1. Field of the Invention
This invention relates to devices for separating vapor and liquid. In particular, the invention relates to separating vapor and entrained liquid in a hydrocarbon distillation tower in which a feedstream is introduced into a flash zone.
2. Discussion of Related Art
Separation units, such as atmospheric distillation units, vacuum distillation units and product strippers, are major processing units in a refinery. Atmospheric or vacuum distillation units separate crude oil into fractions according to boiling point so downstream processing units, such as hydrogen treating or reforming units, will have feedstocks that meet particular specifications. Crude oil separation is accomplished by fractionating the total crude oil at essentially atmospheric pressure and then feeding a bottoms stream of high boiling hydrocarbons, also known as topped crude, from the atmospheric distillation unit to a second distillation unit operating at a vacuum pressure.
The vacuum distillation unit typically separates the atmospheric unit bottoms into gas oil vapors based on boiling point, including light gas oil, heavy gas oil, vacuum gas oil, and vacuum reduced crude. The vacuum reduced crude is also known as residuum or “resid” and leaves the vacuum distillation unit as a liquid bottoms stream.
In atmospheric or vacuum distillation, lighter hydrocarbons are vaporized and separated from relatively heavier hydrocarbons so that they can be fed downstream for catalytic processing. The bottoms separated from crude oil by an atmospheric distillation unit are fed to a flash zone in the lower portion of the vacuum distillation unit. Although the heavier hydrocarbons do not vaporize, they may be carried into the lighter hydrocarbons due to entrainment. The entrained heavier hydrocarbons are typically contaminated with metals, such as vanadium or nickel, which can poison the downstream catalytic processing, such as hydrotreating, hydrocracking, or fluid catalytic cracking.
If the entrainment of the heavier components can be significantly reduced or eliminated, a significant improvement in the quality of the feed for hydroconversion units, catalytic cracking units, and vacuum towers producing valuable gas oil distillates or lube oil distillates can be realized, in both yield and quantity.
Various methods of reducing entrainment of residuum from the flash zone have been developed. Many distillation towers use inlet horns for introducing the feedstream to the flash zone. One type of inlet horn uses a tangential entry for the vapor-liquid feed that opens into a peripheral open bottomed horn. The horn can be an annular or arcuate channel defined by an outer peripheral wall and an internal arcuate wall spaced from the tower peripheral wall and having a closed top. Thus, the feedstream swirls through the horn and the liquid and vapor components impact the walls from centrifugal force and separate since the force on the more dense liquid is substantially greater than the centrifugal force acting on the vapor. The separated liquid flows downward due to gravity to the stripping zone for collection in the bottom portion of the tower. The vapor component also flows downwardly within the horn and then out of the horn toward the lower pressure flash zone and is swept upwardly through the core toward the wash zone of the tower.
One example of a peripheral horn is shown in U.S. Pat. No. 4,770,747 in which the inlet horn has angularly disposed vanes connected between the walls of the channel so that vapor-liquid separation takes place evenly along the arc length of the horn.
Another example of an inlet horn is shown in U.S. Pat. No. 4,315,815 in which corrugated vanes are disposed in the horn for utilizing the centrifugal motion to create turbulence in the stream in the inlet horn. In this case, the turbulence causes a portion of the fine particle size bituminous material to impinge on the surfaces of the inlet horn and recombine with the fluid so that vaporized solvent and steam can be withdrawn.
The problem with the prior art devices is that the inlet horns still allow an amount of vapor with entrained liquid to move up the tower through the core of the horn. In a typical tower, the overflash, which includes vacuum gas oils in the wash oil, is collected from the overflash collection tray and sent to the stripping zone for further processing. It typically has a high percentage of resid and is not suitable for certain feed applications, especially for fluid catalytic cracking (FCC). In order to more effectively use the overflash, especially for FCC feed, it is desirable to further reduce the entrainment of resid. Additionally, more effective de-entrainment will improve the reliability of the wash zone. Excessive resid entrainment in the wash zone accelerates formation of coke, forcing sub optimal operation and shutdown. Higher quality overflash, such as would be acceptable for FCC, can increase wash oil rates and virtually eliminate the risk of coking, allowing units to operate at higher temperatures and higher efficiencies.
Thus, there is a need for a separation device in which entrainment of resid can be significantly reduced.